The present invention relates to methods for stabilizing smectite shale in highwall mining applications.
Highwall mining is one of many methods of surface and subsurface mining used for the extraction of valuable materials such as coal, gold, silver, and other minerals from the earth. Generally, highwall mining involves boring into a highwall of a large pit to extract these materials from a seam. In some instances, the mine may have been mined at different levels (e.g., digging new pits inside pits that have been mined) such that a bench area allows for moving equipment in and out of the newer, deeper pit. Further, having bench areas guards against highwalls becoming too high and unstable. The highwalls are typically high and at a very steep slope, e.g., vertical to about 20% off vertical. FIG. 1 illustrates a highwall mine 100 cross-section having two highwalls 102 with a bench area 104 therebetween, a pit surface 106, and the earth surface 108.
Within highwall mines, the lithology, and consequently the highwall lithology, has volumes that predominantly comprise smectite shale. Highwall mines are typically formed or excavated in areas with a heterogeneous lithology with volumes of smectite shale dispersed therethrough. Consequently, the highwalls have multiple lithologies with portions being smectite shale. Smectite shale is a family of clays that consist of clay platelets having an alumina sheet disposed between two silica sheets, commonly referred to as a 2:1 configuration. Smectite shales are known for swelling caused by water intercalating between the platelets thus expanding the interlayer spacing between the platelets.
Water swollen smectite shale has a lower strength and is believed to be prone to landslides. Heavy rain can exacerbate the landslide risk by saturating the smectite shale. Additionally, the water between the clay platelets may undergo phase changes between ice and water depending on the external temperature of the surrounding environment, which can further destabilize the shale as a result of the size difference between ice and water (i.e., ice crystals have a larger volume than water).
Major wall failures (e.g., landslides, which are often originate with the smectite shale) can occur seemingly without warning, causing loss of lives, damage to equipment, and disruption to the mining process. Efforts to prevent landslides and other major wall failures include highwall strengthening technologies. These may include mechanical stabilization (e.g., with rocks, cement, grout, and concrete) and/or drainage to mitigate erosion. These strengthening technologies are often costly to put into place. Additionally, if an operator needs to move a highwall or dig into a highwall (e.g., to retrieve minerals), these strength technologies are costly and time consuming to deconstruct and reconstruct. Further, these strengthening technologies are preventative measures only and do not address the underlying mineralogy that is most prone to landslides.